Innovative Energy Engineering



Refrigeration

Most cooling requires the application of a refrigeration cycle that cools (evaporator) with the help of an evaporating fluid (refrigerant) and rejects the heat including the compressor energy in a condenser. R410A is one of the most common refrigerants in Building Applications. In general all refrigeration cycle performance and efficiencies depend on ambient and interior space temperature, heat exchanger size, compressor efficiency and control sophistication.

Refrigeration is a process that moves heat from a colder to a warmer medium. Technically all chillers and DX systems are heat pumps since they "pump" heat from the cold medium to the warm medium, against the natural flow. Depending on what the system requires (cooling, or heating, or both) the naming differs. Some equipment can perform both heating and cooling by reversing the flow depending on the mode.

Refrigerant lines should be small enough to allow oil-movement back to the compressor, but should be large enough to reduce pressure drop. Excessive pressure drop costs energy, performance and can cause liquid flashing. Lines should be as short as possible and manufacturer guidelines and design software should be used to accommodate the manufacturer's oil return requirements.

DX Systems

Direct EXpansion systems are the most common in smaller systems ranging from residential sizes to a few dozen tons capacity. The evaporator, where the refrigerant expands, is a cooling coil located directly in the airstream. In most cases the condenser is an air-cooled device, but also can be wet-cooled (like a wet cooling tower) or a water-cooled condenser (in geothermal systems, or water-source heat pumps). Modern systems employ varying capacity (e.g. digital) scroll compressors. Older systems employs single-speed reciprocating compressors, which are noisy, less efficient and have more moving parts. They cycle frequently (wear, inefficiency) and provide unstable Discharge Air Temperature (DAT) and de-humidification. Sometimes inefficient hot gas-bypass is used for capacity control.

DX systems in reverse (condenser inside an air handler, evaporator outside) are Air Source Heatpumps. Some DX systems have a reversing valve that allows operation as cooling or heating device depending on the season, which can be economical in moderate climates. But efficiency at low temperatures is very low and performance diminishes. At low ambient temperature they either require a secondary heating system or are barely more efficient than electric resistance heat.

Chillers

Chillers mostly are used in systems with 200 kW (56 tons) or more cooling capacity. Unlike DX systems, chiller evaporators don't cool an airstream directly, but "chill" a water flow in a direct expansion evaporator. In larger chillers this evaporator is a shell and tube heat exchanger. In small chillers it can be a plate/frame heat exchanger. Heat is rejected in the condenser which can be an air-cooled condenser (remote condenser or outdoor chiller). In larger systems the condenser is water-cooled and the water rejects heat in a cooling tower. The tower can be air-cooled (dry), but most larger systems employ evaporative cooling towers (wet). Chillers are also used to transfer heat from a chilled water loop to a heating loop when simultaneous heating and cooling occurs. This heat-recovery chiller setup saves energy.

Smaller chillers employ scroll compressors, which should vary capacity (i.e. digital scroll). Medium sized chillers often use Helical Rotary compressors (screw)with slide valve or variable speed motors for capacity adjustments. Larger chillers use centrifugal compressors that can vary capacity with varying inlet vanes or varying motor speed. Multiple stages and economizers are employed to improve efficiency and capacity control.

Absorption chillers use water as refrigerant and Lithium-Bromide as absorbent in single or double effect cycles. Instead of a compressor that requires electricity, they use heat and most often a pump to operate the cycle. Absorption chillers are typically very large and require careful design, operation and corrosion protection. In the right application economics can be favorable since electricity consumption is greatly reduced compared to regular chillers. If heat is provided by fossil fuel, economics depend on the fossil fuel cost vs. the saved electricity. But "free" waste, or even solar heat can be used as well.

Ice Storage

During off-peak hours a chiller can cool a glycol solution to below freezing temperatures and freeze water in insulated storage tanks. During the day, when electricity rates are high, the stored ice can chill the glycol solution, which is used in the chilled water coils for cooling. Varying designs either downsize the day chiller, or have ice provide 100% of the day cooling load. With centrifugal chillers it typically is better to place the ice storage upstream of the chiller. For positive displacement chillers the reverse typically is true. However, a detailed energy simulation is required to optimize the chiller plant and ice storage. The colder (than typical chilled water) glycol can help to downsize piping and coil size. Cold air supply systems also work better with ice storage. Ice storage tanks require more space and up-front cost.

Water Source Heat Pumps - WSHP

Unlike air-source heat pumps, multiple WSHP reject heat to a water or glycol loop during cooling mode. In heating mode heat is extracted from said loop. The loop is connected to a a cooling tower (dry or wet) that rejects excess heat if cooling load and compressor heat exceed heating load. If compressor heat and cooling load exceed heating load, a boiler injects heat. A geo-exchange (commonly called geothermal) or surface water loop also can act as heat source or heat sink and improve efficiency. Simultaneous heating and cooling (core area requiring cooling in winter) can make WSHP systems more efficient. Ventilation will require a Dedicated Outside Air System (DOAS) to pre heat and cool (dehumidify) the air. Only arid or cold climates could employ WSHP without dehumidification. Maintenance and noise in occupied spaces is a concern. Efficiency of the small heatpumps and fans is not the best compared to larger equipment. Heating by boiler via heatpump is more costly than using a boiler only.

Variable Refrigerant Flow (VRF)

Similar to WSHP, VRF systems have cooling/heating units in each zone and require dedicated ventilation with dehumidification. But a refrigerant loop connected to a central condenser/evaporator is used in lieu of a water loop. Since no pumps and additional heat exchange are needed, VRF are more efficient than WSHP systems. The large amounts of refrigerant in the spaces (consider ASHRAE 15/34) are of concern and leaks are more likely. Since all heating is done by an air-source heatpump, additional heating systems are required in cold climates. VRF systems also can be combined with water-loops, which can be fed by boilers, geothermal fields and cooling towers. If the building experiences a lot of simultaneous heating and cooling VRF with heat recovery can save energy. One great benefit are space savings since refrigerant lines are very small compared to ducts. However, allowable line lengths are limited. there is added maintenance cost with a proprietary system and maintainable units in occupied spaces. Also controls integration is lacking. VRF typically are considerations when space for ducts is not available, and/or in small applications. An energy simulation is required to judge if energy savings can be accomplished.

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